SH2/SH3 adaptors such as Nck and Crk function by recruiting effector proteins that bind their SH3 domains to sites of tyrosine phosphorylation bound by their SH2 domains. Some forms of Crk can induce cell transformation when highly expressed, and endogenous Crk is implicated in regulating motility and phagocytosis. Nck has been has been shown to mediate localized actin polymerization and plays a role in pathogen invasion and motility. Despite the importance of these proteins and the seeming simplicity of their mechanism of action, little is currently known about the interactions that are important for their biological activity. Because the SH2 and SH3 protein binding domains are relatively promiscuous, both Crk and Nck bind to a multitude of partners in the cell, and the functional importance of any specific interaction or binding partner is largely unknown. Progress is further hampered by the fact that the stimuli that generate phosphorylated binding sites for adaptor SH2 domains have many other effects in the cell, making it difficult to isolate the specific role of the adaptor. Furthermore, the binding partners and ultimate biological effects of the adaptors are highly dependent on their subcellular localization and local concentration. We will use a suite of new experimental tools and approaches developed in the laboratory to manipulate the binding interactions of the adaptors, and the local concentration of the adaptors and their binding partners in the cell. We will identity and determine the subcellular localization of tyrosine-phosphorylated SH2 binding sites for Crk and Nck, identify those Crk SH3- and SH2-binding proteins whose interaction plays a role in Crk transformation, elucidate normal physiological roles for Crk and Nck, and determine the functional relevance of specific protein-protein interactions to those normal roles.